A lithium ion secondary battery has the highest energy density among practical batteries and has been widely used particularly in compact electronic products. In addition, use in automobiles is also expected. For use in these applications, there has been a demand for a lithium ion secondary battery having larger capacity, longer life, and higher safety.
Generally, in the lithium ion secondary battery, an organic separator is used for preventing short circuit between a positive electrode and a negative electrode. An ordinary organic separator is composed of polyolefin materials such as polyethylene and polypropylene, and has physical property of melting at 200° C. or less. Accordingly, when an internal and/or external stimulus causes high temperature of the battery, the organic separator may undergo volume changes such as contraction and melting. Such a phenomenon may cause, e.g., short circuit between a positive electrode and a negative electrode, and discharge of electrical energy, which in turn may cause, e.g., explosion of the battery.
In order to solve these problems, there has been proposed an organic separator and/or an electrode (a positive electrode and/or a negative electrode) having a porous membrane provided thereon containing non-electroconductive particles such as inorganic fine particles. The inorganic fine particles have excellent thermal resistance and have ability to form a porous membrane with high strength (for example, Patent Literature 1 and Patent Literature 2).
However, a preparation of the inorganic fine particles inevitably contains moisture and metal ions, and it is difficult to reduce the content thereof. Contamination of the porous membrane by moisture and metal ions causes an adverse effect on battery performance, so that there is a limitation for further improvement in battery performance with the inorganic fine particles.
In order to solve such a problem, it has been proposed to use organic fine particles in place of the inorganic fine particles (Patent Literature 3 and Patent Literature 4). Since the surface of the organic fine particles can be hydrophobized by selecting monomer species to be used, moisture contamination in the porous membrane can be kept lower than the inorganic fine particles. Further, since the organic fine particles enables metal-ion-free manufacture, contamination amount of metal ions can also be reduced. Therefore, it is expected that use of the organic fine particles realizes further improvement in battery performance.